This invention relates to a process and apparatus for the electrical arc welding of metal members in particular metals or alloys, on which there forms, during welding, a film of refractory oxide, such as aluminum, magnesium and their alloys.
The present invention, while of general application, is particularly well suited for use in welding systems of the type having a refractory electrode which is frequently lodged in an electrically conductive nozzle. The nozzle serves as a counter-electrode through which passes an ionizable fluid. The refractory electrode and the nozzle carry a unidirectional electric current giving rise to an arc which, under the action of the ionizable fluid, forms a column of plasma between the electrode and the nozzle on the one hand and the object to be welded on the other hand.
It is known that the layer of refractory oxide which forms on specific metals, such as those mentioned herein-above, does not permit the utilization of conventional welding processes and apparatus of the type employed to weld metals such as iron and steel, and the heavy metals in general. Certain processes and apparatus have been proposed for permitting the welding of these refractory oxide metals, but they have been subject to a number of disadvantages. The prior processes and apparatus commonly involved either electric arc welding or plasma welding techniques, including those utilizing an electrode lodged in a nozzle confining the arc by means of its outlet orifice. In cases in which a source of direct current was connected between the electrode and the members to be welded, intense bombardment of the electrode took place, resulting in its comparatively rapid deterioration. Equipment utilizing alternating current had the disadvantage that it involved the problem of restriking the arc at each alternation, such as by a supplementary current source for striking a permanent pilot arc between the electrode and the nozzle. In the case of plasma welding techniques, the column of plasma often passed in random or erratic fashion through the nozzle, resulting in instability of the arc.
Some of the attempts to overcome these difficulties involved the use of blown plasma, wherein the plasma produced by the arc "bursts" between the electrode and the nozzle and is projected by ionizable fluid flow supplied at a high rate externally of the nozzle. Primarily because of the direction of travel of the electrons and the electrical neutrality of the workpieces to be welded, however, the prior blown plasma techniques were not particularly effective in overcoming the layer of refractory oxide.
Other attempts to solve these problems effected arcing alternatingly between a negatively charged electrode and a positive workpiece and then between the negative workpiece and a positive nozzle. The arc between the electrode and the workpiece constituted the welding arc, whereas the arc between the workpiece and the nozzle was used to destroy the layer of refractory oxide on the surface to be welded. This technique had several disadvantages, including a relatively complicated electronic circuit and the necessity for periodically interrupting the current to provide the desired alternations. This in turn required a pilot arc for maintaining the arc between the electrode and the nozzle.